The present invention relates, in general, to a method of etch endpoint detection and, more particularly, to a method of detecting the endpoint of the etch of epitaxially grown silicon.
Several techniques are known for endpoint detection of other films typically used in IC processing. Included among these are mass-spectrometric, optical interferometric, and fluorescence detection of a sacrificial coating material (as described in U.S. Pat. No. 4,482,424). These techniques have the common drawback that they do not readily lend themselves to endpoint detection of epitaxially grown silicon.
The mass-spectrometric analysis of an etching plasma is employed to determine resist etch endpoint. For example, when a reaction product typical of the substrate is detected in the mass-spectrometric analysis of the plasma, then the interface between the resist and the substrate is assumed to have been reached, and the etching is discontinued. In the present application there is no resist/substrate interface to be detected. In addition, from the time etching of the substrate starts to the time detection is made of the underlying substrate and etching stopped, part of the substrate is already etched.
In the optical interferometric technique, light is shown on a film undergoing etching; a portion of the reflected light is detected; and the intensity of the detected light is recorded. Etch endpoint is generally detected by looking for sharp changes in slope, or sharp changes in the oscillation frequency, or the detected signal. However, the slope of the output signal does not always change abruptly at the endpoint, particularly if the substrate has optical properties similar to those of the etching film (i.e. when an epitaxial layer is being etched).
The fluorescence detection technique requires that a layer of fluorescent material be placed on the wafer and that a fluorescence detector be used to determine when the fluoresce level has dropped below a certain standard. This would only be applicable if a fluorescence detector were utilized in a production line.